Reduced partially carboxymethylated hen egg lysozyme (approximately 0.8 mol of 1-14C carboxymethyl groups) was air oxidized. The lower (native) and higher hydrodynamic volume forms fractions isolated contained radioactivity, approximately 35% and 65%, respectively. Ion exchange chromatography of the lower hydrodynamic volume forms yielded renatured lysozyme, two major radioactive samples (LHc and LHD) eluting at the positions of monocarboxymethylated lysozyme and two minor radioactive samples eluting at the positions of dicarboxymethylated lysozyme. Samples LHc and LHD contained 0.95 mol and approximately 0.5 mol of 14C carboxymethyl groups respectively. The radioactive tryptic peptides of samples LHc and LHD indicated that all the eight isomers containing three, presumably, native disulfide bonds and one free and one carboxymethylated sulfhydryl group are formed during air oxidation of the reduced partially carboxymethylated lysozyme. Since in each one of these isomers the formation of one of the four native disulfide bonds is permanently blocked, it would follow that no one of the four disulfide bonds of native lysozyme is obligatory in the formation of other three native disulfide bonds. BIBLIOGRAPHIC REFERENCES: Acharya, A.S. and Taniuchi, H.: Formation of the Four Isomers of Hen Egg White Lysozyme Containing Three Native Disulfide Bonds and One Open Disulfide Bond. Proc. Nat. Acad. Sci. U.S.A. 74: 2362-2366, 1977. Acharya, A.S. and Taniuchi, H: A Study of Renaturation of Reduced Hen Egg White Lysozyme. J. Biol. Chem. 251: 6934-6945, 1976.